In the field of vehicle electronics, the use of a high speed controller area network (CAN) is becoming ever more prevalent. In a typical automotive application, the CAN provides a two-wire multiplex communication link that can be routed around the vehicle. Thus, the CAN provides a simple mechanism for a vehicle processing unit to communicate effectively with remote electrical/signal processing units, e.g. vehicle light modules, braking system, airbag modules, etc.
The CAN specifications for road vehicles are defined by the International Standards Organisation (ISO), as described below.                (i) ISO 11898-2: high speed physical layer—part 2;        (ii) ISO 11898-3: low speed fault tolerant physical layer—part 3;        (iii) ISO 11898-4: time trigger CAN; and        (iv) ISO 11898-5: high speed physical layer with low power mode and wake up.        
The CAN specifications support communication over, say, a ten metre length. However, in requiring the CAN bus to support communication over this length, the long wires act as an antenna and as such are effectively subject to automotive electrical transients, as well as industrial transients, such as electro-magnetic interference (EMI) and electro-static discharge (ESD).
Furthermore, as the wires can be very long, the control of the slew rate of signals routed by the wires is also known to be very critical to avoid any EMC emission. In addition, in order to operate the CAN bus successfully, in a problematic vehicle environment; a CAN transceiver must also be able to withstand high voltage transients.
Consequently, it is important to guarantee a good control and matching of the respective devices between the switching of the two CAN wires. To guarantee good matching of the slew rate between the respective wires, the two wires (i.e. the high-side CAN (CAN-H) and the low-side CAN (CAN-L)) must be matched in performance and controlled equally.
A typical CAN driver circuit is illustrated in FIG. 1. The CAN driver circuit comprises a digital transmit input signal 102 that is input to both the CAN-H driver 104 and a CAN-L driver 106. In order to achieve both high-speed and symmetry of operation, low-voltage matched components are generally used. The CAN-H driver utilizes, say, a pnp transistor 130 as an active device operably coupled to Vcc 108, where as the CAN-L driver utilizes, say, a npn transistor 132 as an active device, operably coupled to ground 110.
The respective outputs 118, 120 from the CAN-H and CAN-L drivers 104, 106 are input 114, 116 to a comparator 112. The output from the comparator 112 is a ‘receive’ digital output signal 122. Thus, as will be appreciated by a skilled artisan, the driver circuits that control the signals on the CAN-H wire 118 and CAN-L wire 120 need to be carefully matched, to ensure that the CAN-H driver 104 and CAN-L driver 106 are adjusted to switch between the CAN-H and CAN-L wires 118, 120 in phase. A key aspect to using such a twisted cable is to ensure the current contained in the wires (in both directions) is of equal amplitude and of opposite sign. In this manner, the magnetic field produced, which is proportional to the current, is substantially zero. Thus, electro-magnetic interference due to the current in the twisted pair of cables is minimized. Clearly, any asymmetry between the current in the two wires produces a magnetic field, which is highly undesirable.
In addition, it is known that the common-mode CAN-H bus wire and CAN-L bus wire must also be constant during the switching transition, i.e. when a signal appears on the CAN-H and CAN-L wires, and when it is taken off. Thus, the ΔI needs to be minimized during the transitions otherwise electromagnetic interference is created. In effect, there are two types of common-mode configuration:                (i) Current: where the current value of both the CAN-H and CAN-L wires should be equal but of opposite sign; and        (ii) Voltage: which should be equal, given that the for high speed operation the bus impedance is specified as 60 ohms in the CAN standard.        
Furthermore, to avoid EMC emission, the slew rate applied to signals on the CAN-H bus wire 118 and the CAN-L bus wire 120 must be controlled and matched. The slew rate is a function of the temperature (delay) and of the load. Consequently, the slew rate is difficult to control accurately.
A graphical example of how difficult it is to achieve a good match between the high-side driver and the low-side driver is shown in FIG. 2. Here, the transmit waveform signal 202 is shown with a slight offset to the receive waveform signal 222. If the respective driver circuits are symmetrical, the CAN-H and CAN-L signals in waveform 218 are also symmetrical, resulting in the summation of CAN-H (nominally 3.5V) and CAN-L (nominally 1.5V) values to be flat, at approximately 5V. The summation of the CAN-H and CAN-L signals is often referred to as the ‘common mode’. However, when the driver circuits are not completely matched, the symmetry between CAN-H and CAN-L during transitions is not met. This lack of matching results in a so-called common-mode glitch 230 (say a variation of the order of 120 mV), which is noticeable upon the summation of the CAN-H and CAN-L values.
A solution to this problem is illustrated in the known prior art circuit 300 of FIG. 3, with the use of multiple drivers 316, 318, 320, 322 for the CAN-H and drivers 324, 326, 328 and 330 for CAN-L. A series of very fast switches 336, 338, 340, 342, 344, 346, 348 and 350 are operably coupled to respective serial resistances, where each driver operation is controlled by a fixed delay 304, 306, 308, 310, 312 and 314.
In this regard, the slew rate is fixed by the delay elements 304, 306, 308, 310, 312 and 314, the series resistances and the load capacitance. Accurate selection of these components ensures a good match.
However, with a high voltage range on the output, non symmetrical clamping is (due to the inherent nature of the components) introduced in series with resistance. These high voltage components can be designed to be somewhat symmetrical at low frequencies. However, they will exhibit asymmetry at higher frequencies, say above 100 KHz. Thus, asymmetry of signals between the two wires will generate common-mode glitches at a frequency of above 100 KHz. In effect, a new common-mode (i.e. the summation of the values of the CAN-H and CAN-L wires) exists at each frequency of operation.
European Patent EP 0955750, titled “Line driver with parallel driver stages” by Texas Instruments, as well as European Patent EP0763917 titled “Line driver with pulse shaper” by Lucent Technologies Inc. U.S. Pat. No. 5,194,761 illustrates prior art CAN arrangements.
However, it is recognised that, in order to sustain higher and higher voltage levels during fault conditions, it is no longer possible to use low voltage components. Thus, instead, high voltage components are required to be used, particularly components that exhibit higher parasitic effects. Notably, and problematically, the parasitic components exhibit different characteristics for the high side driver (CAN-H) and the low side driver (CAN-L) outputs. Consequently, the common-mode performance becomes degraded with those high voltage components.
Thus, a need exists for an improved twisted-pair based communication system, apparatus and method therefor, particularly to drive CAN-H and CAN-L bus wires.